Plasmids and the spread of resistance.

Plasmids represent one of the most difficult challenge for counteracting the dissemination of antimicrobial resistance. They contribute to the spread of relevant resistance determinants, promoting horizontal gene transfer among unrelated bacteria. Undistinguishable plasmids were identified in unrelated bacterial strains isolated at huge geographically distant area, with no apparent epidemiological links. These plasmids belong to families that are largely prevalent in naturally occurring bacteria, usually carry multiple physically linked genetic determinants, conferring resistance to different classes of antibiotics simultaneously. Plasmids also harbour virulence factors and addiction systems, promoting their stability and maintenance in the bacterial host, in different environmental conditions. The characteristics of the most successful plasmids that were at the origin of the spread of carbapenemase, expanded-spectrum β-lactamase, and plasmid-mediated quinolone resistance genes are discussed in this review.